Organization of Computers and their Function

（湖南人文科技學(xué)院，湖南 婁底 417000）

【Abstract】： This article mainly introduces the three main components of the computer： the CPU， the memory subsystem， and the I/O subsystem. And a detailed overview of the composition and function of each part. This article provides some guiding significance for follow-up researchers.

【Keywords】： computer， CPU， memory subsystem， I/O subsystem

1 Introduction

It is hard to say exactly when the modern computer was invented. Starting in the 1930s and through the 1940s， a number of machines were developed that were like computer. But most of these machines did not have all the characteristics that we associate with computers today. These characteristics are that the machine is electronic， that it has a stored program， and that it is general purpose.

The third computer generation was also the time when minicomputers became widespread. The most popular model was the PDP-8， manufactured by DEC. Other companies， including Data General Corporation and Hewlett-Packard Company， introduced minicomputers during the third generation.

Finally， since the UNIVAC-1’s introduction， raw computer speed has increased by about 11 to 12 orders of magnitude in about 50 years， or a factor of 10 every five years. This is a truly remarkable achievement. It’s also interesting to contemplate that， if this growth continues over the next 50 years， then by the 100th anniversary of the UNIVAC-1， computers will be operating at speeds on the order of 1023 Flops！

2 Basic Organization of Computers

Most computer systems， from the embedded controllers found in automobiles and consumer appliances to appliances to personal computers and mainframes ，have the same basic organization. This organization has three main components： the CPU， the memory subsystem， and the I/O subsystem. The generic organization of these components is shown in Figure1.

The CPU controls the computer. It fetches instructions from memory， supplying the address and control signals needed by memory to access its data. The CPU decodes the instruction and controls the execution procedure. It performs some operation internally， and supplies the address， data， and control signals need by memory and I/O devices to execute the instruction. Nothing happens in the computer unless the CPU cause it to happen.

Just as the CPU controls the computer （in addition to its other functions）， the control unit controls the CPU. This unit generates the internal control signals that cause registers to load data， increment or clear their contents， and output their contents， as well as cause the ALU to perform the correct function.. The control unit receives some data values from the register unit， which it uses to generate the control signals. This data includes the instruction code and the values of some flag registers.

Data is transferred via the data bus. When the CPU fetches data from memory， it first outputs the memory address on its address bus. Then memory outputs the data onto the data bus； the CPU can then read the data from the data bus. When writing data to memory， the CPU first outputs the address onto the address bus， then outputs the data onto the data bus. Memory then reads and stores the data at the proper location. The processes for reading data from and writing data to the I/O devices are similar.

The internal organizations of ROM and RAM chips are similar. To illustrate the simplest organization， a linear organization， consider an 82 ROM chip. For simplicity， programming components are not shown. This chip has three address inputs and two data outputs， and 16 bits of internal storage arranged as eight 2-bit locations.

Random Access Memory （RAM）， also called read/write memory， can be used to store data that changes. This is the type of memory referred to as X MB of memory in ads for PCs. Unlike ROM， RAM chips lose their data once power is shut off. Many computer systems， including personal computers， include both ROM and RAM.

The CPU treats memory as homogeneous. From the CPU's perspective， each location is read from and written to in exactly the same way. Each memory location performs the same function--it stores a data value or an instruction for use by the CPU. Input/output （I/O） devices， on the other hand， are very different. A personal computer's keyboard and hard disk perform vastly different functions， yet both are part of the I/O subsystem. Fortunately for the system designer， the interfaces between the CPU and the I/O devices are very similar. As shown in Figure1， each I/O device is connected to the computer system's address， data， and control buses. Each I/O device includes I/O interface circuitry； it is actually this circuitry that interacts with the buses. The circuitry also interacts with the actual I/O device to transfer data.

3 Conclusions

In this section we have introduced the CPU from a system perspective， but we have not discussed its internal design. We examine the registers， data paths， and control unit， all of which act together to cause the CPU to properly fetch， decode， and execute instructions. In the systems in this chapter， each byte of data transferred between an I/O device and memory must pass through the CPU. This is inefficient for many common operations， such as loading a program from disk into memory. Direct memory access， DMA， is a method used to bypass the CPU in these transfers， thus performing them much more quickly.

4 References

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